The invention relates generally to the generation of a high brightness optical source through use of a fluorescence concentrator system. The source can be used in a variety of applications, including various medical aesthetic procedures.
As an aid in demonstrating the utility of the invention, it is helpful to review some basic optical concepts. For a pulsed optical source the pulse energy is defined as the average optical power during the pulse multiplied by the pulse width. Typical units for pulse energy are Joules, for power are Watts, and for time is seconds. The optical intensity is the power per unit area, typically expressed in units of W/cm2. The optical fluence is defined as the pulse energy incident on a unit area. Typical units are Joules/centimeter2 (J/cm2). The spectral fluence is the pulse fluence per unit of wavelength. Typical units for wavelength are nanometers so the spectral fluence is often expressed in units of J/(cm2*nm). The brightness of an optical source is the power per unit area per unit solid angle. Typical units are W/(cm2*steradian). The spectral brightness is the brightness per unit wavelength. Typical units are W/(cm2*steradian*nm). Brightness and spectral brightness are also known by the terms radiance and spectral radiance.
It is a well known axiom of optics that the brightness or spectral brightness of a light source generally cannot be increased by propagating the light through any classical optical system. Such optical systems can, at best, preserve the brightness of an optical source. This brightness constraint limits the ability to increase the intensity or fluence by simply focusing an optical beam. Most types of optical sources have insufficient brightness to deliver the intensity or fluence necessary to perform typical aesthetic medical procedures, such as removal of hair, pigmented lesions, or superficial small blood vessels. Thus, special high brightness sources are typically used to perform medical aesthetic procedures.
There are two optical systems that are known to increase brightness. One well-known system is an optically pumped laser. In laser systems it is easily possible to increase source brightness by factor of 10,000 or more. There are many examples of such laser systems, the most common being a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser pumped by a flashlamp or laser diode. These lasers are often incorporated into medical platforms used in aesthetic procedures, for example the Coolglide Xeo system manufactured by Cutera, Inc. of Brisbane, Calif.
A second, much less well-known technique to increase optical brightness is through use of a fluorescence concentrator. A number of papers from the 1950s and 1960s describe concept of a fluorescence concentrator, and it appears that such discussion was largely in connection with different detection systems, for example see the following references which are incorporated herein by reference, Radiance Amplification by Multi-Stage Fluorescence System, W. A. Shurcliff, Journal of the Optical Society of America, Vol. 41, No. 3, p. 209 (1951); The Collection of Light from Scintillation Counters, Richard Garwin, Review of Scientific Instruments, Vol. 31, p 1010, (1961); Radiance Amplification by Fluorescence Radiation Converter, Gunter Keil, Journal of Applied Physics, Vol. 40, p. 3544 (1969).
Fluorescence concentrators guide some fraction of fluorescent light isotropically generated in a body by total internal reflection within the body. The body can be a solid material containing a fluorophore. The guided fluorescent light emerges from an output surface of the body, where the output surface could be one or more ends of the body. The cross-sectional area of the end or ends is typically small compared to the total surface area of the solid. The emerging fluorescent light can have a brightness exceeding that of the original illumination source. The fluorescence concentrators can be arranged serially so that one concentrator pumps a subsequent concentrator, further amplifying the brightness.
Currently fluorescence concentrators are found in a number of commercial systems. One application is as a particle detector in high-energy physics. In this sensor application the illumination source consists of high-energy particles from nuclear interactions. Energy from these particles is partially converted to near-ultraviolet or visible light by a fluorophore contained within an optical fiber. A fraction of the emitted light is transmitted to the fiber end where it is detected by a photomultiplier tube or photodiode. Suppliers for these types of fluorophore doped plastic fibers are Bicron, a division of Saint Gobain, of Newbury, Ohio and Optectron Industries of Les Ulis Cedex, France.
There has been at least one attempt to use a fluorescence concentrator system as an illumination source for an emergency light source. The pump source was a phosphorescent material surrounding a fluorescence concentrator. Such a system was described by Mitsunori Saito & Kazauya Yamamoto, in an article entitled, Bright Afterglow Illuminator Made of Phosphorescent Material and Fluorescent Fibers, found in Applied Optics, Vol. 39, p 4366 (2000). The brightness provided by this source was less than that provided by conventional fluorescent and incandescent lamps. The maximum reported output intensity was slightly less that 100 nW/mm2 or 10 μW/cm2.
As discussed herein one fluorescence concentrator includes a transparent host material containing the fluorophore, the host material forms a geometric solid body within which the fluorophore is contained. This body may be homogenous or may have some internal structure, as will be discussed in more detail below.
A wide variety of light therapies have been developed over the last few decades to treat a number of medical conditions. Several of these therapies make use of light energy for treatment of dermatological conditions, improving the aesthetic appearance of the treated dermis and epidermis. These therapies typically work by selectively heating a naturally occurring (endogenous) chromophore to an elevated temperature sufficiently high to effect and possibly denature the tissue in the region of the chromophore. The therapies require a minimum light intensity or fluence at the tissue. The light should be delivered in a sufficiently short time for selective photothermolysis to occur. That is, the optical energy needs to be applied in a sufficiently short pulse, so that the heat produced by absorption in the targeted chromophore has insufficient time to conduct away from the absorption volume of the targeted tissue. The heat thus locally raises the chromophore temperature, denaturing, or damaging, the tissue immediately surrounding the chromophore. The targeted tissue can then be absorbed, sloughed, exfoliated over time, or otherwise biologically altered through a wound-healing type response by the surrounding tissue leaving the treated area clear or otherwise altered. Selectivity can be achieved by use of optical wavelengths that are strongly absorbed in the targeted chromophore. Selectivity can also be achieved by directing the light only at tissue areas that contain high concentrations of the targeted chromophore. The non-targeted areas are left undamaged or only slightly damaged by the procedure. If is often desirable to cool the epidermis prior to and/or during and/or after the treatment to minimize thermal injury to the non-targeted areas. To achieve therapeutically useful light levels generally the optical intensity must be in the range of, or exceed, 10 W/cm2. This corresponds to, for example, an optical fluence of 0.2 J/cm2 delivered in a 20 msec pulse (many dermatologic anatomical structures are characterized by sizes corresponding to thermal diffusion or relaxation times on the scale of 1–100 ms, such as hair, pigmented epidermal lesions and small-to-medium size blood vessels.) The required efficacious fluence varies widely depending on the target tissue, target chromophore, treatment size, and desired result.
Hemoglobin and melanin are two naturally occurring chromophores which are often targets in light therapies. For example, telangiectasias, commonly referred to as spider veins, and other subcutaneous vascular conditions, are treated by selectively heating the targeted chromophore, hemoglobin, found in blood vessels, with laser light energy. In this procedure selectivity is achieved by directing the treating light at the readily visible blood vessel. Similarly, selective heating of melanin with laser energy is now widely used for hair removal or epilation, removal of skin lesions, and other conditions where melanin bearing structures can be targeted. Both of these therapies may be performed, using a optically-pumped Nd:YAG laser having a wavelength of 1,064 nanometers such as that described in issued U.S. Pat. No. 6,383,176, which is incorporated herein by reference. Such laser-based treatments have been widely adopted, and are successfully treating large numbers of patients for a variety of dermatological and other conditions.
Aside from choosing a therapeutic wavelength which is well absorbed by the target chromophore, it is also important to consider the tissue scattering properties and the absorption of non-targeted chromophore. Hemoglobin has a broad absorption peak located between 500 nm and 600 nm. Melanin has a monotonically rising absorption with shorter wavelengths. Thus, short wavelength treatments that target melanin generally require lower intensities to achieve that same therapeutic efficacy as long wavelength treatments. Tissue scattering also increases with shorter wavelengths. Thus to target shallow structures shorter wavelengths can be used, while longer wavelengths can be used for deeper structures.
Medical apparatus used in light therapy treatments typically have a user-directed applicator or handpiece that delivers light to the skin surface. Typical protocols for dermatologic aesthetic treatments usually involve (1) contacting or positioning the applicator so as to direct treatment light at a local region of skin, (2) optionally initiating some type of epidermal cooling (such as contact with a cooled optical window), (3) directing a pulse of optical energy through the applicator to the skin surface for a proscribed duration, (4) repositioning the applicator to a new treatment region of skin, and (5) repeating steps 2 and 3. This general protocol is referred to as a “stamping” modality.
The stamping modality has many variations. The applicator can be used in either a contact or non-contact mode depending on the applicator design, type of treatment, and cooling method used. If contact is used it can be either “hard” contact, where the applicator is pressed into the skin, or “soft” contact, where the applicator lightly touches the skin. In some cases no contact is required such as the method described in pending U.S. patent application Ser. No. 10/782,534, filed Feb. 19, 2004, entitled METHODS AND DEVICES FOR NON-ABLATIVE LASER TREATMENT OF DERMATOLOGIC CONDITIONS which is assigned to the assignee of the present application and which is incorporated herein by reference. For small isolated features, such as a pigmented skin lesion, only a single treatment site may be required for some treatments. It may be desirable to irradiate the same site with multiple pulses. This can be done by making multiple passes over the treatment area or by directing multiple pulses at a single treatment site before repositioning the applicator.
The light source may be configured in a variety of ways as part of the medical apparatus. A laser or other light source may be contained in a separate console and propagated by some means to the applicator. Other apparatus houses the laser or light source inside the applicator device, avoiding the need to propagate the light from a console. It is possible to configure the medical apparatus as part of a medical platform which allows different apparatus to share common platform components. For example, the Coolglide/Xeo medical platform manufactured by Cutera, Inc. of Burlingame, Calif. can be configured with either a flashlamp or laser light source. Such platforms typically include a means for cooling the skin through the applicator or handpiece. For example, U.S. Pat. No. 6,383,176 and No. 6,485,484 describe one type of cooling used with the CoolGlide laser handpiece, and both of these patents are incorporated herein by reference.
While generally successful, existing optically-pumped laser-based treatments still have certain disadvantages. Specifically, known commercial laser therapy systems have often employed large, rather expensive lasers to generate sufficient intensity or fluence for therapeutic efficacy. Many of these lasers require regular maintenance to provide the desired performance. The lasers typically require precise mechanical tolerances and thus cannot readily be serviced. Additionally, existing lasers are often inflexible in the light wavelengths they produce. As different therapies benefit from different optical wavelengths, entirely separate laser systems are often required to perform different therapies. Finally, laser-based therapies are accompanied by a significant eye safety risk. Eye safety is particularly problematic when there are multiple laser wavelengths and pulse parameters used at a medical site and where multiple spectacle styles and optical characteristics for protective eyewear are required.
More recently, alternative therapeutic light sources have been proposed. These alternative light sources include laser diodes, flashlamps, light emitting diodes (LEDs). While these alternative structures can have significant cost advantages over optically-pumped lasers, each has significant disadvantages. When sufficient laser diodes are combined to generate therapeutic light fluences, the total cost of the device is quite high, often in the many thousands of dollars. Moreover, high power laser diodes are only available in a limited number of wavelengths; they are not currently available in wavelengths shorter than approximately 630 nm and thus cannot target the hemoglobin absorption peak near 550 nm. While flashlamps are very low in cost, they have a large emitting volume and low spectral brightness. Reflectors and apertures are typically used to collect, direct and control the flashlamp light to the dermis. The reflectors must be precisely built and calibrated, as errors can produce hot spots in the spatial energy distribution. Furthermore, as the spectrum of light energy generated by lamps is quite broad and much of the total light energy may disadvantageously cause heating of non-target chromophores. This disadvantageous light can be reduced by use of a wavelength selective optical filter. However, the filter is expensive, reduces the fluence of the desired light, and only imperfectly removes the unwanted light. Both direct flashlamp and filtered flashlamp systems produce light of only moderate spectral brightness over a relatively large area and thus cannot produce a spatially localized, high spectral fluence source. LEDs are low in cost and are available at most wavelengths across the visible spectrum. Unfortunately, LEDs typically have insufficient brightness to cause selective photothermolysis of appropriate tissues or structures bearing naturally-occurring (endogenous) chromophores.
There remains a need for a low cost light source of sufficient spectral brightness and spectral fluence to be therapeutically efficacious in treating various dermatological conditions.